This invention relates to apparatus for the control of spacecraft, and more particularly to a technique for controlling the rotation of the spacecraft.
The invention arose when considering problems associated with the control of a geostationary satellite when it is in its so-called "survival mode", which will be explained later. However the invention could possibly be of value more generally in connection with spacecraft control.
In order to understand the background of the invention it is convenient to refer to FIG. 1 which shows a geostationary satellite in very schematic form. An earth sensor 1, e.g. a camera for viewing the earth, faces in the z direction and pairs of thrusters X+, X-; Y+, Y- and Z+, Z- adjust rotation of the satellite about respective axes. Sun sensors 2 (only one shown) on opposite sides of the satellite provide output signals indicating the direction of the sun relative to the x, y and z axes.
In normal operation of the satellite, as shown in FIG. 2, the z axis is always aligned with the earth, the y axis is parallel with the earth's axis and the x axis makes a tangent with the geostationary orbit. With the satellite stabilized in this way, it is possible to drive solar panels 3 relative to the body of the satellite so that these solar panels 3 remain facing in the direction of the sun as indicated by signals from sun sensors 2.
The field of view of the earth sensor 1 is usually quite narrow since the earth subtends only a small angle at the satellite. For this reason, if the earth is lost from the field of view, it can be difficult to adjust the attitude of the satellite to retrieve it. Unless remedial action is taken, the satellite will quickly fail because the reference direction from which the solar panels 3 are steered to face the sun has been lost and the supply of adequate electrical power will therefore discontinue.
Some satellites have a system which puts the satellite into a "survival" mode if the earth is lost from the field of view of the earth sensor 1. When this survival mode is initiated signals from sun sensors 2 are used to control thrusters Y+, Y- and Z+, Z- so as to align the satellite's x axis with the direction of the sun as shown in FIG. 3. The x axis now provides a new reference direction making it possible to control the satellite's solar panels 3 so that they face the sun, thereby providing the power necessary for the satellite to survive. When the satellite is in this survival mode, intervention can be taken from the ground to retrieve the earth into the field of view of the earth sensor 1.
The time that the survival mode could be maintained would be limited if there were no way of controlling the rate of rotation about the x axis. The conventional way to control rotation about the x axis is to use a gyroscope to sense the rotation and to operate thrusters X+ and X- to control it. However, such a gyroscope needs to be reliable over the entire life of the spacecraft and gyroscopes having this degree of long-term reliability are costly. The invention arose when considering possible solutions to this problem.